Hydraulic cements, i.e., inorganic cements that harden or set under the influence of water, are frequently used in cementing operations associated with oil, gas, water and brine wells, as well as dam and tunnel construction. For instance, aqueous hydraulic cement slurries are used, during or after the completion of the drilling of an oil or gas well, to fill the annulus between the borehole wall and the outside of the casing. Usually, such wells are cemented by pumping a cement slurry downwardly though the casing with a shoe and/or float valve and then upwardly into the annulus surrounding the casing. The cement (a) provides a sheath surrounding the casing that prevents or inhibits communication between the various formations penetrated by the well, (b) aids in bonding and supporting the casing, (c) protects the casing from corrosion, (d) prevents blowouts by quickly forming a seal, (e) protects the casing from shock loads in drilling deeper, and (f) aids in sealing off zones of lost circulation.
Hydraulic cements manufactured for use in oil and gas wells are subject to wide ranges of temperature and pressure when in position in a well and differ considerably from cements used at atmospheric conditions. As a result, specifications covering eight classes of oil well cements, designated Classes A, B, C, D, E, F, G and H, are provided by the American Petroleum Institute (API). These cements comprise portland cement and a variety of cementing additives, such as those discussed below. Portland cement used in the cements classified by API are primarily comprised of about 40 to about 60% tricalcium silicate, about 15 to about 30% B-dicalcium silicate, about 8 to about 12% tetracalcium aluminoferrite and about 3 to about 8% tricalcium aluminate, with the total of tricalcium silicate and dicalcium silicate generally being about 75 to about 80% (Free CaO and MgO are generally held below about 1.5% and about 5%, respectively). Other hydraulic cements used in such wells are aluminous or pozzolanic cements. Cements and cementing are described by D. K. Smith in Cementing, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. (1976).
In many uses of hydraulic cements, it is necessary for an aqueous cement slurry to be emplaced within or next to a porous medium, for example, earthen strata in the cementing of oil and drilling wells. When such is the case, water filters out of the slurry and into the strata during the setting period. When this occurs to an appreciable extent there usually results an uncontrolled setting rate, improper placement, impaired strength properties and contamination of the surrounding strata. The loss of fluids into the surrounding strata may be controlled by incorporation of fluid loss control additives or agents into the hydraulic cement. Fluid loss control additives for use in such cements include copolymers of N,N, dimethylacrylamide and 2-acrylamide, 2-methyl propane sulfonic acid as described by Rao et al in U.S. Pat. No. 4,515,635, modified alkylenediamine or polyalkylenepolyamine compositions as described by Willis et al in PCT International Publication No. WO 85/01935, and polysacharides such as HEC as described by Hook in U.S. Pat. No. 3,483,007.
Use of synthetic polymers as fluid loss agents is expensive. Therefore, there has been a desire to use less expensive natural polymers such as HEC. Although HEC provides good fluid loss, there is still a desire to reduce costs by replacing HEC with a more efficient and less expensive fluid loss additive, which does not adversely effect the rheological characteristics of the cementing composition or slurry.